Ionicity in Ionic Liquids: Origin of Characteristic Properties of Ionic Liquids

Physicochemical properties of 1-ethyl-3-methylimidazolium ionic liquids having [CF3CO2]−, [CH3SO3]−, [CH3CO2]−, [CF3SO3]−, [BF4]−, and [N(CF3SO2)2]− anions were measured precisely. The ionicity, in other words the dissociativity of the ionic liquids, was estimated from the molar conductivity ratios (Λimp/ΛNMR), where Λimp is molar conductivity from ionic conductivity measurements and ΛNMR is that from ionic diffusivity by pulsed-gradient spin-echo nuclear magnetic resonance (NMR) measurements. The ionicity, Λimp/ΛNMR, was controlled by the magnitude and balance of Coulombic forces, which were mainly altered by the anionic Lewis basicities, and van der Waals forces. The Λimp/ΛNMR values of the ionic liquids ranged from 0.5 to 0.8 and followed the anion-dependent order [BF4]− > [N(CF3SO2)2]− > [CF3SO3]− > [CF3CO2]− > [CH3SO3]− > [CH3CO2]−, which is the inverse order of the anionic basicities or the order of acidities of their conjugate acids. Calibration curves for the Lewis basicity–Λimp/ΛNMR relationship are presented for the 1-ethyl-3-methylimidazolium, [C2mim]+, and 1-butyl-3-methylimidazolium, [C4mim]+, ionic liquids, from which either Λimp/ΛNMR or Lewis basicity can be estimated if either of them is known. Viscosities of various [C2mim]+ and [C4mim]+ ionic liquids were correlated with the effective ionic concentration Ceff, which was estimated from the ionicity and molar concentration of these ionic liquids. With increasing Ceff, the viscosities tended to increase, although at the same Ceff, [C4mim]+ ionic liquids showed higher viscosities than [C2mim]+ ionic liquids because of the enhanced van der Waals forces.